Removed unnecesary Qspi stuff for now. Compiles fine for samd51 and samd21. Have no samd21 platform or tpuart to do thorough testing but nothing fundamental was changed in samd_platform other than the ifdef checks.
Now checking for __SAMD21__ and __SAMD51__ in knx_facade, samd_platform (samd21) and samd51_platform (samd51)
Preparation for external flash functionality in platform.h